Magnetic Field Compensation Apparatus for Cathode Ray Tube

A cathode ray tube comprising:

• a glass envelope having a rectangular faceplate panel and a tubular neck connected thereto by a funnel;
• an electron gun positioned in the neck for directing electron beams toward the faceplate panel;
• a yoke positioned in the neighborhood of the funnel-to-neck junction, the yoke having windings configured to apply a horizontal barrel shaped field and a vertical pincushion shaped field to the beams, the horizontal barrel field shape being adjusted to give an optimized spot shape at sides of the screen, causing an overconvergence of the beams at the sides of the screen;
• at least one magnetic field sensor located near the glass envelope for sensing an ambient magnetic field environment of the CRT;
• a controller receiving a signal from the magnetic field sensor;
• register correction coils being mounted in the vicinity of the neck and being dynamically controlled by the controller to shift the beams;
• and,
• quadrupole coils applied to the neck and having adjacent poles of alternating polarity such that the resultant magnetic field being dynamically controlled by the controller based on the magnetic field sensor signal moves outer ones of the beams to correct a misconvergence caused by the register correction coils, the quadrupole coils also being dynamically controlled by the controller to correct overconvergence at the sides of the screen caused by the yoke.